Duct jointing systems comprising a plurality of elongate frame members along with a similar number of corner pieces adapted to be received endwise into the frame members, are well known. Exemplary of the prior art in this area are the following patents:
U.S. Pat. No. 3,712,650, Mez, issued Jan. 23, 1973; PA0 U.S. Pat. No. 4,252,350, Smitka, issued Feb. 24, 1981; PA0 U.S. Pat. No. 4,218,079, Arnoldt, issued Aug. 19, 1980; PA0 U.S. Pat. No. 3,923,326, Mez, issued Dec. 2, 1975; PA0 U.S. Pat. No. 4,244,609, Smitka, issued Jan. 13, 1981; PA0 DE OS No. 2434160, Pedall, filed July 16, 1974.
The prior art, as exemplified by the foregoing publications, generally discloses a duct jointing system which includes a plurality of elongate frame members, each frame member being a single piece of sheet metal folded to define a section including an open channel for receiving an edge portion of one wall of a sheet metal duct, a first web extending generally perpendicularly to the channel from a location adjacent the channel-space, an outer portion at the end of the first web remote from the channel, the outer portion extending in the same direction as the channel, and a second web extending obliquely towards the channel from the remote end of the first web. Also included in the known duct jointing system are clip means for clipping adjacent frame members together, and corner pieces with angled ends, each end being insertable endwise into one end of an elongate frame member. In particular, U.S. Pat. No. 4,252,350, Smitka, incorporates these features.
Despite all of the developments set forth in the patents identified above, there is continuing pressure in the industry to reduce the cost of the duct jointing system still further, and it is an aspect of the present invention to accomplish this.
One approach to reducing the cost of a duct jointing system of the kind described is to use the thinnest metal possible for the basic frame members which receive the end edges of the duct walls. However, there is a minimum thickness required for at least certain portions of the frame member, in order to ensure adequate strength. It must be appreciated that the frame members are called upon not merely to hold the open ends of adjacent duct portions in juxtaposition. In many installations, the ducts are called upon to carry air or gas which is substantially above the ambient pressure, and this tends to "bow out" the end edges of the duct walls. The frame members of the duct jointing system must of course withstand this tendency to warp outwardly, which increases with increasing internal pressure. Under standard building codes, there are maximum joint deflection requirements. The joint must be strong enough not to exceed these maximums, and therefore the metal must be of a certain minimum gauge.
We have discovered, however, that only certain portions of the section of the frame member require this minimum thickness of sheet metal, because these portions are either in compression, or are bent or folded in such a way that the fold constitutes a weakened location requiring the additional strength of the minimum sheet metal thickness. Other portions of the section, however, are purely in tension, and especially where these portions are straight in section, it would be possible to utilize a thinner sheet metal material.
Naturally, it is impractical to try to provide a sheet metal blank which has different thicknesses corresponding to these different requirements, since the expense of differential rolling would more than offset any saving in material.
It is therefore another aspect of this invention to provide a frame member for a duct jointing system which has the minimum thickness required for areas which either are under compression or support complicated folds, while allowing a lesser amount of material in the areas which are under pure tension, without requiring differential rolling of the sheet metal blank.